Reactors for conducting Fischer-Tropsch reactions and other exothermic reactions to produce a range of higher-weight reaction products, such as higher olefins, higher paraffins, and wax are well known. However, these reactors do not provide the robust control over reaction and heat constraints that are needed to perform syngas conversion reactions and other exothermic reactions that produce a range of lower-weight hydrocarbon reaction products, such as lower olefins. There is a need in the art for reactors that provide the control over reaction and heat constraints required to perform such reactions and produce and collect lower-weight reaction products.